Differences in the mechanism of action of two neuroleptic drugs, haloperidol and clozapine, are studied in mice following acute administration and during and after chronic treatment. The dopamine concentration and utilization in different brain areas (striatum olactory tubercle, cortex, and substantia nigra) are measured. Changes in presynaptic and postsynaptic receptors are examined after discontinuation of chronic dosage to evaluate the development of tolerance or sensitization.